In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior to, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby.
Both D.C. and A.C. type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating D.C. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an A.C. corona generating potential is applied to the conductive wire electrode and a D.C. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
Some of the problems with these devices are that the wires have a propensity for vibration, singing and sagging, especially when they are used for charging over a long distance perpendicular to the process direction, and it is difficult to use them placed closed to the surface to be charged. Also, they are sensitive to breakage as well as difficult to mount.
Various approaches to answering these problems have been tried in the past. For example, U.S. Pat. No. 3,711,710 discloses a corona charging system wherein a plurality of ion discharging coronodes, made from thin conductive strips, which charge the surface of a receiving medium within a document reproduction machine. U.S. Pat. No. 3,959,690 is directed to a corona charging element for an electrophotographic reproduction machine that includes a coronode member in the form of a metal strip which charges a conductive photoreceptor surface by discharging an ion charging current directly onto the photoreceptor surface. An electrostatic precipitator apparatus is disclosed in U.S. Pat. No. 4,349,359 that includes an ion generating electrode in the form of a long twisted strip that charges the surface of collecting plates. U.S. Pat. No. 4,626,876 discloses a solid state corona discharger that includes a pair of parallel strip-shaped A.C. electrodes that discharge an ion charging current onto the surface of a photosensitive member. A particle charging apparatus is shown in U.S. Pat. No. 4,414,603 that includes a group of parallel, narrow strip-shaped corona electrodes which collectively discharge an electric field to within a designated charging space when activated by an A.C. source voltage. Although these attempts at solving the above-mentioned charging problem have had some success, they have not been entirely satisfactory.
From a printing point of view, industry has desired to provide a reliable, high resolution non-contacting printing system. One approach to this end is ion projection printing which, in one form, entails depositing electrostatic charges in a latent image pattern directly upon a charge receptor surface and then rendering the charge pattern visible, in some known manner. Clearly such a system would have decided benefits in machine design as compared to the known contact printing arrangements, as it would overcome the primary contact printing problem of friction and mechanical wear. Typically, ion projection printing comprises the generation of ions in an ion stream and the control of the ions which may reach a charge receptor surface.
Numerous ion generating devices are available for printing or charging purposes. For example, in U.S. Pat. No. 4,463,363 there is taught a D.C. air breakdown form of ion generator. In U.S. Pat. No. 4,524,371 a fluid jet assisted ion projection printing apparatus is disclosed that includes a housing having ion generation and ion modulation regions. A bent path channel, disposed through the housing directs and transports fluids with ions entrained therein adjacent an array of modulation electrodes which control the passage of ion beams from the device. Generation of charged particles in U.S. Pat. No. 4,155,093 is accomplished by extracting them from a high density source provided by an electrical gas breakdown in an electrical field between two conducting electrodes separated by an insulator. A corona discharge unit is used in conductive toner transfer in a copier in U.S. Pat. No. 4,174,170. The corona discharge unit includes a slit to permit transfer of conductive toner particles onto a copy paper charged by the corona unit. The distance between the slit and a corona wire is 5 mm. U.S. Pat. No. 3,396,308 discloses a web treating device for generating a flow of ionized gas. This device includes an opening through which the gas is directed towards a receptor surface. An elongated hollow housing 11 has tapered sides 14 terminating in a pair of lips 15 which form a narrow and elongated slot 16. U.S. Pat. Nos. 3,598,991 and 4,100,411 show electrostatic charging devices including a corona wire surrounded by a conductive shield. In the U.S. Pat. No. 3,598,991, a slit 13 is formed in the shield to allow ions to flow from wire 12 to a photoconductive surface 2 to deposit an electric charge thereon. In the U.S. Pat. No. 4,100,411, a pair of lips 16 and 17 define a corona ion slit 18. Japanese Patent Document No. 55-73070 discloses a powder image transfer type electrostatic copier that includes a corona discharge device having a slit in a shield plate. In Japanese Patent Document No. 54-156546 a corona charge is shown having a plurality of grating electrodes in the opening part of a corona shield electrode. U.S. Pat. No. 4,763,141 discloses a printing unit that includes a current limited corona wire located 1-5 mm away from biased conductive plates which form a slit that allows ions to pass therethrough onto a receptor surface. All of the above devices are incorporated herein to the extent necessary to practice the present invention. These devices have not been entirely satisfactory in that they are costly, some of them are hard to fabricate and most are inefficient.